Previously, many types of optical recording media have been developed for laser writing. For example, an article in Optical Engineering, Vol. 15, No. 2, March-April, 1976, p. 99 discusses properties of a large number of media. Some of these media require post write processing before they can be read, and some can be read immediately after laser writing. The media of interest herein are for "direct read after write" capability, commonly known as "DRAW" media. Presently known laser DRAW media are thin metal films in which holes may be melted, composite shiny films whose reflectivity at a spot may be reduced by evaporation, thin films of dyes or other coatings which can be ablated at a spot, and dielectric materials whose refractive-index may be changed at a point, causing a scattering of light when scanned with a read laser.
The most common DRAW media are thin metal films, usually on a glass substrate. Thin metal films have several advantages: First, they can be produced easily in small quantities with commercially available sputtering equipment. Second, they can be read either by reflection or by transmission. Third, films of tellurium and bismuth have relatively high recording sensitivities.
Fortunately, for all of these reasons, metal films have enabled a large amount of research to be conducted and progress to be made in the design of optical data storage systems. To date, tellurium has evolved as the most widely used of the metal films. However, tellurium must be manufactured by an expensive, batch-type, vacuum sputtering technique; it does not form a tenacious coating; and it introduces manufacturing and environmental complictions because of its toxicity and since it rapidly oxidizes in air it must be encapsulated in an airtight system in order for it to achieve an acceptable archival life.
What is particularly desirable about tellurium is that it has a low melting temperature for a metal, 450.degree. C., and also a very low thermal conductivity of 2.4 watts per meter per degree Kelvin at 573.degree. K. In comparison, silver metal has a melting temperature of 960.degree. C. and a thermal conductivity of 407 watts per meter per degree Kelvin at the same elevated temperature. When these two metals are considered for laser recording with short pulses of laser power, the tellurium is far superior from a recording sensitivity standpoint since the low thermal conductivity keeps the heat generated by the laser beam confined to a small area and the lower melting temperature facilitates the melting of the hole. Conversely, silver metal, because of its high thermal conductivity, about 170 times that of tellurium, would not normally be considered suitable for laser recording.
Attempts have been made to improve the laser recording sensitivity of various types of metal layers. In U.S. Pat. No. 3,911,444 Lou, Watson and Willens disclose a vacuum-deposited metal film recording media for laser writing incorporating a separately deposited plastic film undercoat between the metal film and a flexible transparent substrate to thermally insulate the metal layer in order to require less energy to write with a laser.
Although it is possible to produce reflective metallic coatings of many types on substrates by vacuum sputtering or evaporation, silver is relatively unique in that it can also be created by photographic techniques and, in particular, by silver diffusion transfer. In U.S. Pat. No. 3,464,822 Blake discloses a silver diffusion transfer reversal process for creating electrically conducting silver images for the fabrication of printed circuit boards. That invention, in turn, is based upon silver diffusion transfer process inventions of the reversal type, leading to black non-reflective and nonconductive images, one example being U.S. Pat. No. 2,500,421 by Dr. E. H. Land. The silver diffusion transfer reversal process forms the basis of direct positives by the Polaroid Land process of Polaroid Corporation and the Gevacopy and Copyrapid processes of Agfa-Gevaert. These reversal processes should be distinguished from the silver diffusion negative process. One such process leading to black non-reflecting and non-conducting images, is described in U.S. Pat. No. 3,179,517 by Tregillus. A silver diffusion transfer negative process is used in the present invention.
It is well known that if very small, high electrical conductivity metal spheres or spherical particles are distributed through a dielectric medium, the effective dielectric constant or refractive index will rise owing to the added induced dipoles of the metal particles. For the case of homogeneously distributed particles, see Principles of Microwave Circuits, edited by C. G. Montgomery, McGraw Hill Book Company, Inc., 1948, pp. 376-397.
Previously, a reflective silver laser recording medium was the subject of a prior patent application Ser. No. 012,235 by J. Drexler. In that application, a processed black silver emulsion was converted to a reflective recording medium by heating at least to 250.degree. C. until a shiny reflective appearance is achieved.
An object of the invention was to devise a non-toxic, reflective DRAW laser recording and data storage medium which may be manufactured without the use of a vacuum system and on a continuous basis and which may be used to record low-reflective spots in a reflective field with relatively low energy laser pulses. Another object was to devise a reflective laser recording and data storage medium which permits the pre-recording of control indicia and certain data base data by photographic means to facilitate the use of discs or plates in both the recording apparatus and the playback apparatus. Another object was to permit replication of optically recorded media by photographic contact printing on a rigid or flexible substrate that can be read in reflection or transmission. Another object was to devise a laser recording and data storage medium which could be fabricated from commercially available photoplates and films or minor modifications thereto, to achieve low cost. Another object was to devise a laser recording medium which does not require a high temperature processing step and therefore will permit the use of ordinary, low-cost photographic plastic film bases or other available plastics as substrate materials permitting fabrication of recording discs with center holes by a low-cost stamping operation. Yet another object of this invention was to devise a single-step silver diffusion transfer photographic process which could produce a highly reflective electrically non-conducting surface layer having a thickness of 1 micron or less contained almost entirely within the gelatin or colloidal carrier.